Design is one of those things that is debatable when it comes to successful blogging. For example, Steve Pavlina has one of the ugliest blogs I have ever seen, but his content makes up for it in spades. But as easy as it has become to make a great-looking blog, there is no reason not to put a little time and money into making your blog look as sharp as possible. If your blog is pleasing to the eye, it will keep your readers on the site a little longer, resulting in more pages viewed and more links clicked.
There are lots of tricks to good design, but they all start with the theme. If wordpress or drupal or typepad is the engine and drivetrain, your theme is the body and interior of your publishing vehicle. You could piece together your own Ford Escort which will get you from point A to point B, but with a little effort you could be cruising along in something more stylish, sporty or elegant.
My advice is to invest in a solid theme from a solid theme designer. Woo themes is one of the most innovative theme design companies out there. And the best part is the quick support. Speaking from my experiences with different designers, I have found that a great theme is nothing without great support. Woo themes excels on both points.